Originally Posted by
gsa103
Mark, thanks for doing that calculation. I had been meaning to do a similar calculation but didn't have a chance yet, you saved me the trouble. The stiffness numbers can probably also be calculated from backing out the geometry used by Giant for their tests or VeloNews and their tests.
It seems like every time some does a calculation they come up with a number <1% of power is the maximum potential loss. What this really says is that a modern frame is stiff enough to not matter, and that in the overall scheme of things stiffer isn't automatically better, if it means adding weight or reducing ride quality. At the same time, if you can make a frame stiffer there's enough benefit to make a difference.
The real question now is how much loss is there from a frame design like a Roubiax, where the rear triangle is allowed a large amount of flex and is heavily damped?
Answer to in bold is Specialized engineers apparently decided that Roubaix had enough performance 'loss' in the rear triangle to redesign the bike...new SL4 Roubaix has a completely new rear triangle similar to the Tarmac. Now I own a SL3 Roubaix and will tell you that redesign of the rear triangle wasn't needed because the SL3 is the best bike I have ridden but they incurred the R&D and retooling cost anyway. So why did they do that do you suppose? 1% improvement? I doubt it. Also, why would they take a softer rear triangle which is the signature design parameter of the Roubaix with longer chainstays and thoughtfully designed seat stays for compliance and make it stiffer thereby making the ride quality more harsh which is reported with the SL4 Roubaix on big hits? Seems to belie the intended purpose of the bike doesn't it. Apparently Specialized engineers believed it was important enough to improve the Roubaix in the direction of Tarmac for
greater performance at the detriment of comfort over hard strikes to the frame. So how did they restore a modicum of ride quality? By reducing the stiffness of the seatpost. This isolates the rider more from the firing down force to the pedals I wrote about earlier. Btw, Trek engineers did the same thing with the Domane which could be argued is done even better...separating vertical and horizontal deflection. The lateral stiffness of the Domane is said to be equivalent to or greater than the Madone. This is why top riders race it.